While practically only homo-polyethylene terephthalate (PET) is used to produce polyester fibers and filaments, virtually exclusively PET-based copolyesters (COPET) that are modified with from 2 to 5 and sometimes up to 7 mol % of cocomponents are now used to make PET bottles for carbonated sweetened beverages and the like.
The resultant advantages at lower processing temperatures, because of lower melting points, are a low acetaldehyde content and glass-clear transparency, because of a lower crystallization rate. Known cocomponents include comonomers of the compound classes of dicarboxylic acids (or methyl esters thereof), diols, and oxycarboxylic acids.
For bottle grade COPET, isophthalic acid, naphthalene-2,6-dicarboxylic acid, 1,4-cyclohexanedimethanol and diethylene glycol, which also forms in small quantities in homo-PET production in situ, are preferably used (for instance, see European Patent Disclosure EP-0 041 035). These comonomers are added to the two main components, that is, terephthalic acid or dimethylterephthalate and ethylene glycol, at the beginning of the production process, or in other words upon esterification or transesterification. Adding them at the end of PET production leads to a drastic decrease in viscosity which makes processing questionable. Hence in the prior art, PET granulates for bottles and for textile applications have not been able to be produced simultaneously in the same esterification and polycondensation equipment, even though the problem of how to later deluster PET melts for textile purposes has now been solved (German Patent DE40 39 857 C2).
The prior art provides various points of departure for producing copolyesters from PET melts, but the object of producing a lower-melting linear, i.e. unbranched, statistical or random-distribution copolyester with simple equipment, rapidly, and without changing the intrinsic viscosity (I.V.) has not been attained at all or at most only partly. (The I.V. is a conventional standard number for the molecular weight for bottle polyesters.)
In European Patent Disclosure EP-0 422 282 A1, a process is described in which the dianhydride of an aromatic tetracarboxylic acid, preferably pyromellitic acid dianhydride, is metered into a PET melt and worked in. The resultant granulate is distinguished primarily by a very rapid increase in I.V. in solid phase polycondensation at even relatively low temperatures, because the substance added links the polymer chains in an additional polyaddition reaction. The result in principle is a copolyester; because the quantity of pyromellitic acid anhydride, 0.6 weight %, is optimal in view of post condensation, the influence on the melting point is only 2.degree. C., as long as isophthalic acid is not additionally employed as a cocomponent in the starting material for the process.
Finally, the dianhydride also has the side effect that the second carboxyl groups liberated after the addition reaction to the anhydride groupings can at least sometimes lead to branching in a subsequent reaction. As a result, although the so-called melt strength in extrusion blow molding is increased, nevertheless at the same time the flow properties are worsened.
In the process according to German Patent Disclosure DE 41 31 362 A1, the usual dicarboxylic acids and diols are used as cocomponents, specifically in the form of a low-viscosity pre-copolymer with a high proportion of comonomer, which is mixed with homo-PET melt. In this process, to produce the prepolymer, one or two additional reactors and also a mixing extruder for mixing the prepolymer with the homopolyester are required. After the preferred melt mixing time of 5 to 15 minutes, there is still no copolyester present in this process, but rather only a physical polymer mixture for granulation, which still has virtually the same high melting point as the homo-PET. The copolyester, not formed until the solid phase post condensation, is still not entirely statistical, because its melting point is up to 2.degree. C. higher than the melting temperature of conventionally prepared, genuinely statistical copolyesters of the same compositions. The explanation for the copolymer formation mechanism in this process can be found on page 3, lines 44-51 of this document: first, after a sufficiently long melt dwell time, only block copolyesters form, which during the post condensation approach closer to a random cocomponent distribution in the molecule chains, the lower the molecular weight of the admixed copolyesters with the comonomer. A further disadvantage of this process is that there is a lower limit to the I.V. value and hence to the molecular weight of the comonomer-copolyester, which is dictated by the economy of the entire process, because the polymer mixture is reduced in its I.V. value compared with the homo-PET.
U.S. Pat. No. 4,680,345 describes a process with which elastic polyesters can be made, by continuously carrying out a polyaddition reaction in a reaction vessel between melted aromatic polyesters, especially polybutylene terephthalate, and lactones in relatively large proportional quantities. The reaction conditions given are a temperature range of 210.degree. to 260.degree. C. and a reaction time of 30 minutes to 6 hours. The result is a block copolymer with soft segments of polylactone, which lend elasticity, and hard crystalline segments of the aromatic polyester. Besides the disadvantage of the long reaction time, the difficulty in this process is that gas cushions of unreacted lactone vapor form in the upper part of the reactor, and if additional provisions are not taken (positioning the reactor vertically or obliquely or incorporating gas baffles), problems occur and the quality becomes worse.
Reactions between polyester and lactones are also described in the following eight references:
In the Derwent Abstract No. AN 94-238850, J 061 72507 is cited describing an elastomeric copolyester with improved heat resistance obtained by reacting an aromatic polyester with a lactone compound, wherein a diol component of the aromatic polyester has a special formula. Nothing in particular is stated regarding the production method, and such a copolymer combination would not be suitable for producing bottles.
In the Derwent Abstract No. AN 93-121484, J 050 59192 is cited describing a continuous production method for elastic polyester films and sheets. Here, aromatic crystalline polyesters are continuously subjected to an addition-polymerisation in a double-screw extruder with lactone compounds from which sheets are directly formed after a short retention time of maximally 30 minutes at maximally 280.degree. C. However, the advantage of the high quality of the sheets must be "paid for" by the use of a very expensive machine, which is used as the reactor and for devolatizing unreacted lactone. Furthermore, in the course of this method additional tin or phosphorous catalysts are preferably added to the polyester which, however, would not permit a use of the resultant films or sheets in the field of food packaging. Furthermore, under the conditions recited in the exemplary embodiments no statistical, but instead block copolyesters (hard blocks of aromatic polyesters and soft polylactone blocks), are created. This is clear from the ratio between the amount of caprolactone and the relatively small melting point decrease of the polyester, which correlates with the elastic properties.
In the Derwent Abstract No. AN 90-159117, J 020 99555 is cited describing copolyester on a PET basis with up to 5 weight-percent of beta-methyl-delta-valerolactone (MVL) as the comonomer and up to 50 weight-percent of crystallization accelerator. Production takes place conventionally by esterification and polycondensation (with ring opening polymerisation of the lactone during polycondensation), and it is used for highly crystalline injection-molded pieces showing no transparency.
In the Derwent Abstract No. AN 90-159116, J 020 99554 is cited describing a copolyester which is analogous to the one disclosed in J 020 99555 with the sole difference being that in place of MVL up to 2 weight-percent .epsilon.-caprolactone is used.
In the Derwent Abstract No. AN 90-152364, J 020 97519 is cited describing copolyesters which mainly contain terephthalic acid and ethylene glycol as well as up to 5 weight-percent of MVL Manufacture takes place conventionally and fibers, films and bottles are named as applications.
In Derwent Abstract No. AN 90-152365, J 020 97520 is cited describing copolyesters analogous to J 020 97519 with the sole difference that in place of MVL up to 2 weight-percent of .epsilon.-caprolactone are incorporated during polymerisation.
In Derwent Abstract No. AN 85-078330, J 600 31525 is cited describing a highly viscous block copolyester which is created in the solid phase by a reaction between a homopolyester (for example PET) and a lactone (for example .epsilon.-caprolactone). Such block copolyesters still have a high melting point which is similar to the homopolyester and therefore do not permit lower processing temperatures, such as would be necessary for bottle production.
In Derwent Abstract No. AN 84-259558, J 591 57117 is cited describing a modification of a crystalline aromatic polyester (for example PET or PEN=polyethylene naphthalate) with 2 to 5 weight-% of lactone, for example .epsilon.-caprolactone, by reaction at 180.degree. to 260.degree. C. in the presence of an organic tin, aluminium or titanium catalyst with the optional use of a solvent. The properties resulting therefrom make the polyester suitable for films, sheets and vessels.